The present invention relates to a colour display tube system and more particularly to a method of correcting dynamic electron beam misconvergence in a colour display tube.
It is known to correct for static convergence errors occurring particularly in in-line electron gun display tubes by means of a series of pairs of pre-magnetized rings mounted externally on the neck of a display tube in the vicinity of the centering cup. The pairs of rings are pre-magnetized as two-, four- and six-pole magnets respectively, and by rotating the rings of each pair relative to each other static convergence errors can be corrected. In an alternative arrangement such errors can be corrected by at least one permanently magnetizable ring disposed in or around the tube neck or mounted on the electron gun so as to surround the three beam paths. Initially the convergence errors are detected and subsequently the magnetizable ring is magnetized as a multipole, the number and strength of the poles being selected in response to the particular convergence errors detected. A suitable magnetizing method is disclosed and claimed in British Patent Specification No. 2.000.635 B (PHN 8845).
When a deflection yoke is mounted on a tube neck, even one having coils wound to a tight production spread, it has been found that misconvergence of the electron beams occur as the beams are scanned, for example raster scanned, across the faceplate. Various theories have been put forward as to why such misalignments occur and in those cases where saddle coils are used the actual laying down of the wires in each coil may not be the same from coil to coil and also it is believed that bends in the coils could give rise to dynamic convergence errors.
It is an object of the present invention to correct for dynamic convergence errors in a colour display tube.